Semiconductors are continuously diminishing in size. Corresponding to this size reduction is an increase in the power densities of semiconductors. This, in turn, creates heat proliferation problems which must be resolved because excessive heat will degrade semiconductor performance. Heat pipes are known in the art for both transferring and spreading heat that is generated by electronic devices.
Heat pipes use successive evaporation and condensation of a working fluid to transport thermal energy from a heat source to a heat sink. Heat pipes can transport very large amounts of thermal energy in a vaporized working fluid, because most working fluids have a high heat of vaporization. Further, the thermal energy can be transported over relatively small temperature differences between the heat source and the heat sink. Heat pipes generally use capillary forces created by a porous wick to return condensed working fluid from a heat pipe condenser section (where transported thermal energy is given up at the heat sink) to an evaporator section (where the thermal energy to be transported is absorbed from the heat source). Heat spreader heat pipes can help improve heat rejection from integrated circuits. A heat spreader is a thin substrate that absorbs the thermal energy generated by, e.g., a semiconductor device, and spreads the energy over a large surface of a heat sink.
Heat pipe wicks for cylindrical heat pipes are typically made by wrapping metal screening of felt metal around a cylindrically shaped mandrel, inserting the mandrel and wrapped wick inside the heat pipe container, and then removing the mandrel. Wicks have also been formed by depositing a metal powder onto the interior surfaces of the heat pipe, whether flat or cylindrical, and then sintering the powder to create a very large number of intersticial capillaries. Typical heat pipe wicks are particularly susceptible to developing hot spots where the liquid condensate being wicked back to the evaporator section boils away and impedes or blocks liquid movement. In many prior art heat pipes, this hot spot effect is substantially minimized by maintaining the average thickness of the wick within relatively close tolerances.
Powder metal wick structures in prior art heat pipes have several well documented advantages over other heat pipe wick structures. One draw back to these wicks, however, is their relatively low effective thermal conductivity compared their base metal, referred to in the art as their “delta-T”. Traditional sintered powder metal wicks have a thermal conductivity that is typically an order of magnitude less than the base metal from which they are fabricated. In a conventional smooth wick heat pipe, there are two modes of operation depending upon the heat flux at the evaporator. The first mode occurs at lower heat fluxes, in which heat is conducted through the wick with the working fluid evaporating off of the wick surface. The second mode occurs at higher heat fluxes, in which the temperature gradient required to conduct the heat through the relatively low conductivity wick becomes large enough so that the liquid contained in the wick near the heat pipe enclosure wall becomes sufficiently superheated that boiling is initiated within the wick itself. In this second mode, vapor bubbles are formed at and near wall/wick interface and subsequently travel through the wick structure to the vapor space of the heat pipe. This second mode of heat transfer can be very efficient and results in a lower over all wick delta-T than the first, conduction mode. Unfortunately, the vapor bubbles exiting the wick displace liquid returning to the evaporator area leading to premature dry out of the evaporator portion of the wick.
Ideally, a wick structure should be thin enough that the conduction delta-T is sufficiently small to prevent boiling from initiating. Thin wicks, however, have not been thought to have sufficient cross-sectional area to transport the large amounts of liquid required to dissipate any significant amount of power. For example, the patent of G. Y. Eastman, U.S. Pat. No. 4,274,479, concerns a heat pipe capillary wick structure that is fabricated from sintered metal, and formed with longitudinal grooves on its interior surface. The Eastman wick grooves provide longitudinal capillary pumping while the sintered wick provides a high capillary pressure to fill the grooves and assure effective circumferential distribution of the heat transfer liquid. Eastman describes grooved structures generally as having “lands” and “grooves or channels”. The lands are the material between the grooves or channels. The sides of the lands define the width of the grooves. Thus, the land height is also the groove depth. Eastman also states that the prior art consists of grooved structures in which the lands are solid material, integral with the casing wall, and the grooves are made by various machining, chemical milling or extrusion processes. Significantly, Eastman suggests that in order to optimize heat pipe performance, his lands and grooves must be sufficient in size to maintain a continuous layer of fluid within a relatively thick band of sintered powder connecting the lands and grooves such that a reservoir of working fluid exists at the bottom of each groove. Thus, Eastman requires his grooves to be blocked at their respective ends to assure that the capillary pumping pressure within the groove is determined by its narrowest width at the vapor liquid interface. In other words, Eastman suggests that these wicks do not have sufficient cross-sectional area to transport the relatively large amounts of working fluid that is required to dissipate a significant amount of thermal energy.